Why isn't the SET operator evaluated remotely?

You can check the operands and check data source restrictions to determine why the SET operator is not evaluated remotely.

Considerations:

Are both of its operands completely evaluated at the same remote data source? If the answer is no and it should be yes, examine each operand.
Does the data source have any restrictions on this SET operator? For example, are large objects or long fields valid input for this specific SET operator?